The research proposed for the second year of this support will be centered on the role of the properdin system as a potential modulator of the immune response to cellular antigens. Having completed the preparation of monospecific antisera to rat and mouse properdin, Factor B and C3, these reagents will be employed to monitor the fate of these reactants during the immune response to cellular antigens. Methods are now being developed to estimate the immunoglobulin response on a weight specific basis to sheep erythrocytes and to a transmissible tumor cell line in syngeneic rats. The availability of the latter will be particularly useful in evaluating the role of the several immunoglobulin classes as modulators of host resistance. The evaluation will be undertaken in terms of tumor cell cytotoxicity and/or growth-enhancing potential of tumor cell specific antibodies on a weight specific basis.